Adult dental caries, including both coronal and root caries, is an infectious disease that afflicts the majority of Americans aged 55 and older and is the most common chronic disease at midlife. In addition, adult caries exacts a significant economic toll, and this economic toll continues to grow. Despite the high prevalence and bacterial pathogenesis of caries, no FDA-approved anti-microbial treatment for caries is available to the American dental professional. The Prevention of Adult Caries Study (PACS) is a randomized, double-blind, placebo-controlled Phase III clinical trial conducted under FDA Investigational New Drug license #45,466. This study is designed to evaluate the efficacy of a topical, temporary, 10% w/v chlorhexidine dental coating in reducing caries increment in at-risk adult dental patients. This study will follow 1000 patients over a 13- month study period at four centers with vastly different populations. The primary endpoint for this study will be caries increment. The centers participating in this proposed research are: Center for Health Research, Kaiser Permanente Northwest, in Portland, Oregon;Tufts University Dental Clinic in central Boston, Massachusetts;Dental Service of Massachusetts Clinical Center in Southborough, Massachusetts, and;the dental clinic of the Tuba City Regional Health Care Corporation in Tuba City, Arizona. Kaiser Permanente's Center for Health Research in Portland, Oregon will act as the data-coordinating center, and Tufts University will act as the Administrative Center.